1. Field of the Invention
The invention concerns a rotating machine control command method, a servocontrol system for implementing said method, and a rotating machine provided with a system of this kind. To be more precise the present invention concerns a method of controlling the torque and the stator flux of a rotating machine.
2. Description of the Prior Art
Vectorial rotor flux control devices are known in themselves. Their control function is based on actual control of the magnetic state of the rotor and the torque of the machine. However, this type of device necessitates the provision of sensors on the rotating parts of the machine in order to sense a mechanical quantity needed for the calculation. Also, these devices are highly sensitive to variation in the parameters of the machine. Finally, these devices necessitate the use of pulse width modulation (PWM) which introduces a tim-edelay into the response time of the machine on the occasion of a torque or speed step.
In an alternative method known in itself the control quantities are the electromagnetic torque and the stator flux. This method does not necessitate the use of PWM.
This method relies on vectorial modelling of the machine and the voltage inverter.
For the machine. it is known that the electromagnetic torque is a function of the angle between the rotor flux rotating vector and the stator flux rotating vector and the moduli of these flux vectors. In other words, the electromagnetic torque is a function of the vector product of the rotating flux vectors: EQU T.sub.em =K(.phi..sub.R .times..phi..sub.S)
The stator voltage vector V.sub.s is delivered by a three-phase voltage inverter, each phase including a single-pole two logic levels (SP2LL) switch. Accordingly, the stator voltage vector V.sub.s can assume eight (2.sup.3) states V.sub.1 . . . V.sub.8, two of which V.sub.1, V.sub.8 are of zero amplitude (zero states) in the stator fixed coordinate system (.alpha.,.beta.), according to the combination of the states of the three SP2LL of the inverter.
A direct torque control (DTC) system known in itself (FIG. 1) relies, in the stator coordinate system (.alpha.,.beta.), on maintaining the modulus .vertline..phi..sub.S .vertline. of the rotating stator flux vector .phi..sub.S in a hysteresis band H and on controlling the torque T.sub.em by accelerating the rotating stator flux vector .phi..sub.S relative to the rotor flux vector .phi..sub.R for an increase in the torque T.sub.em (increase in the angle between the two flux vectors) and by stopping the stator flux vector .phi..sub.S so that the rotor flux vector .phi..sub.R catches up to reduce the torque (reduction in the angle between the two flux vectors).
The stator flux vector .phi..sub.S is controlled by means of a finite table. For a given location N.sub.i (i=1 . . . 6) of the rotating stator flux vector .phi..sub.S in the plane of the stator (.alpha., .beta.), this table contains the states V.sub.1. . . V.sub.8 of the stator phase voltage vector V.sub.s which enable stopping of the stator flux vector (zero states V.sub.1, V.sub.8) and those which are used to open out the angle between the flux vectors .phi..sub.S, .phi..sub.R, whilst maintaining the stator flux vector .phi..sub.S in the hysteresis band H. The finite table below and FIG. 1 illustrate this method.
______________________________________ Finite table T.sub.em increasing T.sub.em .vertline..phi..sub.S .vertline. .vertline..phi..sub.S .vertline. decreasing ZONE N.sub.i increasing decreasing .phi..sub.s stopped ______________________________________ N.sub.1 V.sub.3 V.sub.4 V.sub.1, V.sub.8 N.sub.2 V.sub.4 V.sub.5 V.sub.1, V.sub.8 N.sub.3 V.sub.5 V.sub.6 V.sub.1, V.sub.8 N.sub.4 V.sub.6 V.sub.7 V.sub.1, V.sub.8 N.sub.5 V.sub.7 V.sub.2 V.sub.1, V.sub.8 N.sub.6 V.sub.2 V.sub.3 V.sub.1, V.sub.8 ______________________________________
In FIG. 1, for example, .phi..sub.S is in the zone N.sub.6 and its end tracks the state V.sub.2 of the stator phase voltage vector V.sub.s. The state V.sub.2 is a state which causes .vertline..phi..sub.S .vertline. to increase. The system will switch to V.sub.3 as soon as .vertline..phi..sub.S .vertline. reaches the upper limit of the hysteresis band H.
This technique has a number of drawbacks, including:
The finite table is not exhaustive in terms of the possible dynamic situations of the rotating machine.
Conceiving a table to take exhaustive account of the situations of the machine is inconceivable (in the simple case of an inverter with three SP2LL there are already six domains in each of which four states may be used) . What is more, a malfunction of one of the SP2LL eliminates three possible states of the stator phase vector V.sub.s which significantly reduces the size of the command finite table and can end up with the rotating machine in an out of control situation (torque peak).
The proposed technique dedicates stator flux .phi..sub.S control (maintaining the stator flux modulus within the hysteresis band H) to torque T.sub.em control. Configurations in which stator flux .phi..sub.S control is required concomitantly with torque T.sub.em control are not provided for.
In the case of a negative torque step (stepwise torque reduction) at low rotor rotation speed, the response dynamics of the above solution are very poor. In particular, the response time to the negative step is in the order of four times the response time to a positive step of the same amplitude.
In a torque reduction configuration, the device stops the rotation of the stator flux (zero state) and the rotor flux catches up the stator flux, so reducing the angle and therefore the torque. At low motor rotation speeds, the rotor flux also rotates at low speed, which significantly affects the torque response dynamics.
This configuration at low rotation speeds can be illustrated by the starting up of a traction unit, the poor dynamics by slippage of the traction unit wheels (torque reduction too slow).
The switching strategy for the voltage inverter is the same under transient conditions (the instantaneous values of the flux and of the torque are far away from the commanded values) or under steady state conditions (the instantaneous values of the flux and of the torque oscillate about the commanded values). Accordingly, under steady state conditions, the average switching frequency of the voltage inverter is no longer controllable and is chaotic. For high maximal voltages and currents this can quickly lead to irreversible damage to some SP2LL and therefore end up with the rotating machine in an uncontrollable state, for the reasons previously cited.
One aim of the present invention is to propose a method in which, the control quantities being also the electromagnetic torque and the stator flux, regardless of the situation of the rotating machine at a given time, the optimal stator phase voltage vector V.sub.s state is chosen, from all the possible states, for the best possible response to the required control strategy.